Interchangeable Lenses

To have interchangeable lenses on SLR cameras, the lenses must be designed to accomodate the physical restraints of the camera body. SLR bodies have a physical requirement for a certain amount of space between the detector and the back of the lens. this space is occupied by the reflex mirror. The reflex mirror sits in the lightpath from the lens and intercepts that light and sends it to the viewfinder. The mirror also provides some light for exposure and autofocus functions. This mirror has to move out of the lightpath in order for the image to be exposed. The distance from the detector to the lens mount is called the "flange-focal distance" and varies a bit by SLR brand.

Flange focal distances of various brands:

Nikon F - 46.5 mm

Olympus OM - 46 mm

Pentax K - 45.5 mm

Canon EF - 44 mm

Canon FD - 42 mm

Olympus 4/3 - 38.7 mm

A lens designed for a Nikon mount will focus an image at infinity 46.5 mm behind the flange, i.e. at the detector. If I were to put an Olympus 4/3 system lens on a Nikon body the image from infinity would be short of the detector at 38.7 mm behind the flange. I will lose the ability to focus at infinity. That doesn't mean that I won't get an image at all. I will get an in-focus image of objects at closer range where the shorter object distance translates into a slightly longer image distance. The take home point is that you will lose infinity focus when trying to use any lens from a system with a shorter flange focal distance than the one you are using at the time. Canon lenses don't adapt well to Nikon bodies.

Conversely, you can use lenses from systems with a longer flange focal distance on a body with a shorter flange focal distance. You will just need to add a little more distance between the lens and the detector to get infinity focus. This could be achieved by an adapter or by changing the focus of the lens. You can use Nikon lenses on most every other SLR body with the proper adapter and retain the ability to focus at infinity.

There is a second factor in the ability to adapt lenses across camera systems and that is the diameter of the lens mount. That can make adapting lenses that should work otherwise unworkable. I won't go into more on this since it doesn't really impact the main discussion of flange distance.

That's why there are Nikon lens to Canon body adapters out there (the Canon mount diameter is larger than the Nikon allowing the Nikon lens to fit slightly into the Canon mount) and few Canon lens to Nikon body adapters.